385176 Dispersed Microenvironments for Chemical Evolution
Biopolymers in current living organisms are usually synthesized with the help of enzymes and cell compartmentalization. Such mechanism was unlikely to exist in the prebiotic earth. Therefore, the first biopolymers for the evolution of early life forms should come from simpler systems with abiotic reaction mechanisms. Miller-Urey experiments have demonstrated the possibility of the formation of amino acids and hydroxy acids in the prebiotic world.1 Those materials can be used as building blocks for biopolymers. However, even though the source of monomers is understood, the formation of polymers in the prebiotic ocean still faces a number of challenges. The high water concentration in the ocean can hamper the polycondensation reaction by shifting the equilibrium to the reactant side. This enhances the difficulty of forming long chain polymers and shortens their life time.
In this work, an oil/water biphasic reaction media is designed to solve this problem. It has been shown that some water immiscible organic solvents can exist in the prebiotic world.2 Because of the lower water concentration in those oil solvents, they can facilitate the polymerization reaction by shifting the equilibrium to the product side and protect the synthesized polymers from hydrolysis by water. Also, the increased hydrophobicity of the growing polymer chain can further concentrate those polymers in the oil phase and speed up the evolutionary process. The oil phase can also be dispersed and stabilized by surfactants to form an emulsion. The dispersed oil phase has much higher interfacial area and can facilitate the absorption of building blocks from the prebiotic ocean.
The present work first focuses on the polymerization of lactic acid in the oil/water biphasic system. Polyesters have been proposed as one possible ancestor for polypeptides3 and lactic acid is a significant product from the Miller-Urey experiments. Also, polylactic acid (PLA) is a biodegradable polymer that has been well characterized and can be used in drug delivery applications.
Two different abiotic reaction pathways have been studied in this work. The first one is the polycondensation reaction of lactic acid with dicyclohexylcarbodiimide as condensing agent and 4-dimethylaminopyridine as catalyst. It has proved to be an efficient condensing agent for the polymerization of lactic acid when organic solvent is used alone.4 In our work, the interfacial reaction between the hydrophobic carbodiimide and hydrophilic lactic acid is able to produce long chain PLA. The second reaction pathway is starting from the ring-opening polymerization of lactide, the cyclic dimer of lactic acid. 5 Lactide has higher solubility in organic solvents than lactic acid. Therefore, the reaction can proceed directly in the oil phase. The ring-opening polymerization can be catalyzed by 4-dimethylaminopyridine which is a weak organic base and its distribution between oil and aqueous phase can be controlled by the pH of water. Polyester with high yield can be obtained and preserved in the oil phase under near neutral pH and room temperature.
The findings in this work provide another methodology to solve the mystery of the origin of life. The presence of oil phase successfully facilitates the polymerization and extends the life time of the synthesized polyester. The same biphasic reaction system can also be applied to the reaction and evolution of other biopolymers in the future.
1. Miller, S. L.; Urey, H. C., Organic compound synthesis on the primitive earth: Several questions about the origin of life have been answered, but much remains to be studied. Science 1959, 130 (3370), 245-251.
2. Lasaga, A. C.; Holland, H. D.; Dwyer, M. J., Primordial oil slick. Science 1971, 174 (4004), 53-55.
3. Orgel, L., Some consequences of the RNA world hypothesis. Origins Of Life And Evolution Of The Biosphere 2003, 33 (2), 211-218.
4. Akutsu, F., Inoki, M., Uei, H., Sueyoshi, H., Kasashima, Y., Naruchi, K., Yamaguchi Y., Sunahara, M., Synthesis of poly(lactic acid) by direct polycondensation of lactic acid using 1,1'-carbonyldiimidazole, N,N,N',N'-tetramethylchloroformamidinium chloride and N,N'-dicyclohexylcarbodiimide as condensing agents. Polym Journal 1998, 30, 421.
5. Coulembier, O., Dubois, P., 4-dimethylaminopyridine-based organoactivation: From simple esterification to lactide ring-opening "Living" polymerization. Journal of Polymer Science Part A: Polymer Chemistry 2012, 50 (9), 1672-1680.